1. Field of the Invention
The invention relates to the technology of fusion of glass, and particularly to techniques wherein the conductivity of molten glass is utilized to develop the energy required for fusion, so that thermal energy is developed within the molten glass itself. More particularly, the invention relates to an electric fusion process wherein the current is supplied to the bath of molten glass by electrodes disposed vertically on the hearth of the fusion furnace.
2. Description of the Prior Art
Electrical energy is used for a wide variety of specific purposes in the fusion and refining of glass. In many cases this use of electrical energy is relatively peripheral. An example is local reheating of the bath of molten glass in order to promote certain convection patterns, to eliminate bubbles, or to improve temperature uniformity in large baths.
Outside of these very narrow specific cases, the use of electrical energy for fusion proper is often a mere adjunct of much earlier techniques wherein fusion is accomplished by means of burners. This is even more substantial when the production capacity is increased.
Despite a number of factors which are difficult to gauge and master, the experiences gained from burner-heated furnaces has yielded satisfactory process conditions with respect to product quality. Electric fusion technology has been developed with the focus on reproducing the conditions of burner-heated systems, even if this does allow advantage to be taken of the special characteristics of this new mode of furnace heating.
Large capacity burner-heated furnaces customarily are long and shallow. The charge of starting materials is introduced at one end of the furnace and the molten glass is withdrawn at the other. The thickness of the layer of charge over the bath, which varies as fusion proceeds, is on the average relatively small. The temperature varies widely in different regions of the bath, being low near the hearth and near the charging end, and being high at the surface and at the tapping end.
The choice of electric fusion substantially alters these conditions which are linked in one way or another to the use of burners and to the mode of heat transfer imposed by burners.
In particular, in an electric furnace with vertical electrodes, the charge of starting materials is incorporated in the bath of molten glass without lateral displacement of said charge over the surface of the bath, and the entire surface can be covered by the charge. In this case, if the entire surface is effectively involved in heat and mass transfers between the bath and the floating charge, one may substantially reduce the surface area of the electric furnace with respect to that of a furnace of the same capacity heater by burners.
The invention relates to this type of furnace in which the surface of the melt is covered by the charge of starting materials. This type of furnace is accordingly called a "cold arch" furnace. A need therefore continues to exist for a method by which an electrical fusion furnace can be more effectively used in the manufacture of glass.